Great improvements in label printing

Based on the work of Zikzakmedia, I extended the label printing capabilities of OpenERP. My goal was to provide quick high-volume product label printing.

The following improvements were made:
-Better usability like label language selection and default label format for every template.
-Refactoring to make it possible to easily add printing engines.
-Added a mako-RML engine to the existing mako-XSL:RML engine that has more possibilities in label design and eliminates an extra step that could be source of errors.
-Added a mako-SLCS engine to print to Bixolon label printers.
-Added an editable grid to fill in with products and label quantities to print plus a button on pickings to automatically fill in this grid from their data.

The modules that provide these functionalities:
-label: made by Zikzakmedia in stable 5.0 extra addons. Provides PDF printing with XSL:RML and RML.
-label_product in stable 5.0 extra addons
-label_slcs in stable 5.0 extra addons
-For label_slcs it is useful to install the printjob module from community addons that could send the generated commands directly to the printer. It's made by Pegueroles SCP & NaN.